1. Field of the Invention
The present invention relates to a photoelectric sensor provided with light-projecting elements and a light-receiving element.
2. Description of the Related Art
A photoelectric sensor is used for detecting the presence of an object in a detection region, the position of the object, the color of the object, etc. by means of projecting light on the detection region and receiving the reflected or transmitted light from the object.
In a separate type photoelectric sensor, a sensor head portion for projecting light and receiving light and a main body portion for processing various signals are separated from each other.
As a related-art separate type photoelectric sensor, there is a photoelectric sensor including: a sensor head portion including a light source, and a light-receiving element; and a main body portion including various circuits. The circuits in the main body portion are connected to the light source and light-receiving element in the sensor head portion by two electric wire cables.
The light emission state of the light source of the sensor head portion is controlled from the main body portion side through one of the electric wire cables. With light emission from the light source, light reflected on an object is received by the light-receiving element of the sensor head portion. A signal output from the light-receiving element is transmitted to the main body portion through the other electric wire cable.
As another related-art photoelectric sensor, there is a photoelectric sensor including: a main body portion including a light source, and a light-receiving element; and a sensor head portion connected to the main body portion by two optical fiber cables (e.g. see Japanese Patent Laid-Open No. 330940/1999).
In the photoelectric sensor, light emitted from a light-emitting element of the main body portion is led to the sensor head portion through one of the optical fiber cables. Light reflected on an object is led from the sensor head portion to the light-receiving element of the main body portion through the other optical fiber cable.
The related-art photoelectric sensors however have the following problems.
In the photoelectric sensor in which the main body portion and the sensor head portion are connected to each other by two electric wire cables, a signal for controlling the light emission state of the light-emitting element and a signal output from the light-receiving element are transmitted by two electric wire cables adjacent to each other and having a length of about 2 m to about 10 m. For this reason, the signals propagated through the two electric wire cables interfere with each other. As a result, the level of noise increases, so that S/N (signal-to-noise ratio) deteriorates. Accordingly, detection accuracy is lowered, so that reliability is lowered.
On the other hand, in the photoelectric sensor in which the main body portion and the sensor head portion are connected to each other by two optical fiber cables, light emitted from the light-emitting element of the main body portion is transmitted to the sensor head portion through one of the optical fiber cables while light reflected on the object is led from the sensor head portion to the light-receiving element through the other optical fiber cable. For this reason, attenuation of the quantity of light caused by transmission of light becomes remarkable. When, for example, the quantity of light is attenuated to about 65% by one optical fiber cable, no more than about 42% of the quantity of light emitted from the light-emitting element can be received.
As described above, in the photoelectric sensor in which the sensor head portion and the main body portion are connected to each other by two optical fiber cables, light-utilizing efficiency is low.